The problem I see so many times is people want expects in the field to work for free, or for the “experience.”
My favorite – “Need editor/cameramen/colorist/fill-in-the-blank, to help me on this unbelievably cool project of mine. There is no pay but the experience to work with me is more than enough. Please send resume, demo reel and references to…”
But Steve, if you are offering real training to the person this is exactly what an intern program is for. But as others have said, they will often slow you down but if you find a good one you can both profit from the relationship. As long as you are truly giving something to other person it is very ethical. Best way to do this is to work with them (train them) while not on the set. That way they have a little experience to go on.